1. Field of the Invention
The present invention relates to a piezoelectric sensor for sensing acceleration or vibration and, more particularly, to a piezoelectric sensor to be mounted on a substrate such as a printed circuit board.
2. Description of the Related Art
An acceleration sensor which is a kind of piezoelectric sensor is usually used by packaging it or mounting it on a mounting substrate such as a circuit board which has a circuit for processing acceleration signals generated by the sensor.
Mounting of an acceleration sensor on a mounting substrate has caused two major problems in the prior art.
The first problem is as follows. The design of an acceleration sensor to be mounted on a substrate is made such that the sensor senses acceleration acting in directions perpendicular to the plane of the mounting substrate or acceleration acting parallel to the plane of the mounting substrate, depending on factors such as the use of the sensor and specifications of the circuit board. It is therefore necessary that the acceleration sensor be mounted on the circuit board so as to be sensitive to acceleration acting in either one of the above-mentioned directions.
Meanwhile, the acceleration sensor has a rectangular parallelopiped configuration having six surfaces, and is adapted to be jointed to the circuit board at a specific one of the six surfaces or a surface opposite to this surface. This is because the acceleration sensor has a flattened rectangular parallelopiped configuration having a rectangular bottom and top surfaces and side surfaces of a height which is small as compared with the two sides of the rectangle comprising the top and bottom surfaces, so that the sensor can be soldered to the circuit board while being stably held on the circuit board. For instance, the height ranges from 0.5 to 0.7 times as large as the length of the shorter side of the rectangle defining the top and bottom surfaces.
Consequently, it has been necessary to prepare two types of acceleration sensors: an acceleration sensor of the type which is capable of sensing acceleration acting perpendicularly to the plane of the circuit board and an acceleration sensor of the type which is capable of sensing acceleration acting parallel to the plane of the circuit board. An acceleration sensor of either one of these two types is selected for use. This not only raises the costs of production of acceleration sensors but also requires costs for storing and administrating these two types of acceleration sensors, resulting in a rise in the costs of various products incorporating such acceleration sensors.
The second problem is as follows: The circuit board on which an acceleration sensor is packaged may be deflected for any reason, before or after the packaging of the acceleration sensor. specifically, the deflection of the circuit board causes a corresponding deformation of the acceleration sensor. Such a deformation of the acceleration sensor due to an external force adversely affects the piezoelectric member which senses acceleration, thus hampering sensing of acceleration. This problem is particularly serious when the acceleration sensor is intended to sense acceleration acting perpendicularly to the plane of the mounting substrate, because in such a case the direction of acceleration to be sensed coincides with the direction of deflection of the mounting substrate. Consequently, the acceleration sensor may be influenced by the deflection of the circuit board so as to erroneously produce an acceleration signal even when there is no acceleration acting on the sensor.